Constructed mangrove wetland as secondary treatment system for municipal wastewater
Introduction
Wetland treatment systems have been considered as an alternative to conventional treatment methods, especially for small communities (Soukup et al., 1994, Kivaisi, 2001, Solano et al., 2003), because of their low treatment cost and low maintenance, especially in suburban or rural areas without any large-scale sewage treatment facilities. In the past few decades, constructed wetlands have been applied in treating municipal (Gumbricht, 1992, Kaseva, 2004, Chung et al., 2008), industrial (Mays, 2001, Jacob, 2004, Gottschall et al., 2007), agricultural and livestock wastewater (Comín et al., 1997, Knight et al., 2000). The feasibility of employing constructed wetlands as a secondary treatment system is attractive because it saves land areas; however, the subject still needs more research. Most of the existing constructed wetlands use perennial plants such as common reeds, cattails and bulrush, which are strongly recommended to be harvested periodically (Hammer, 1989, Cronk, 1995, Gopal, 1999). Frequent harvesting would lead to poor plant growth and increase the cost of manpower (Cronk, 1995, Gopal, 1999). Hammer (1989) therefore suggested that other species of perennial plants should be used to avoid annual plantings.
Mangrove swamps commonly found along tropical and subtropical coastlines, usually between 35°N and 35°S latitude, are some of the most productive wetlands in the world (Tam and Wong, 2000, Al-Sayed et al., 2005). In previous decades, mangroves in many countries have been seriously damaged by human activities, and ecological restoration of mangroves through mangrove planting has been commonly used to reverse such losses in the last decade worldwide (Lewis, 2005). Because of their high productivity, mangrove plants have a large demand for nutrients and many mangrove habitats struggle with a nutrient deficiency problem (Li, 1997, Alongi et al., 2005). The beneficial effects of nutrient addition on growth of mangroves have been reported (Clough et al., 1983). Mangrove plants that have colonized the inter-tidal habitat are also well known for their ability to withstand various environmental stresses, including high salinity, waterlogging, alternating aerobic and anaerobic conditions and unstable substratum (Clough et al., 1983). Chiu and Chou (1993) reported that Kandelia candel could transfer oxygen from its aerial parts to its roots, and a portion of oxygen would be leaked from the roots into the adjacent soil, producing a thin aerobic zone surrounding around the plant roots. The very extensive root systems of mangrove plants thus create a significant aerobic zone in the rhizosphere for oxidation (Chen et al., 1995). In addition, the mangrove soil and roots were found to harbor diverse groups of microorganisms which played essential roles in nutrient transformation (Al-Sayed et al., 2005). These unique functions of mangrove ecosystems create a suitable environment for removing and transforming pollutants in wastewater. Some previous studies have demonstrated that mangrove wetland systems made a significant contribution to the removal of nutrients and organic matter from wastewater (Sansanayuth et al., 1996, Wong et al., 1997, Chu et al., 1998, Tam et al., in press). However, there are problems with using mangrove wetlands for nutrient removal. One of them is the need of frequent tidal flushing, which makes the use of mangroves less practical. It will be more cost-effective if the mangrove wetland treatment system can be operated without regular tidal cycles. Our recent research has demonstrated that mangrove plants can grow well in fresh water without tidal flushing (Tam, 2004). The present study aims to determine the performance and efficiency of constructed subsurface-flow mangrove wetlands, without tidal flushing, as a secondary treatment system. The role of mangrove plants K. candel in treating primary settled municipal wastewater under two hydraulic retention times (5- and 10-day HRTs) will also be investigated. Among eight true mangrove plant species found in Hong Kong, K. candel is the most widely distributed and common species. It can be found in all mangrove swamps, while for the two genera in the same family as Kandelia, Bruguiera is present in 28 out of 44 mangrove swamps and Rhizophora is absent in Hong Kong (Tam and Wong, 2000).
Section snippets
Experimental set-up
The mangrove wetland treatment microcosms consisted of 12 PVC tanks. Each microcosm had the dimensions of 0.67 m (L) × 0.54 m (W) × 0.38 m (D). Individual microcosms were divided into three zones, inlet (0.12 m × 0.54 m × 0.15 m), treatment (0.45 m × 0.54 m × 0.15 m) and outlet (0.10 m × 0.54 m × 0.15 m), and filled with different media (Fig. 1). In inlet and outlet zones, the media consisted of gravel (around 2.27–2.91 cm in diameter), while the treatment zone was filled with soil collected from Sai Keng, a typical
Dissolved organic carbon
The mean concentration in the influent was 72.79 mg L−1 with temporal fluctuations (Fig. 2). The effluent concentrations in the planted microcosms were lower than the unplanted ones at both retention times. The DOC removal was generally higher in treatments under longer retention time; for instance, in planted microcosms, 70.43% and 76.38% removal were obtained at 5- and 10-day HRT, respectively (Table 2). According to the results of two-way ANCOVA (Table 3), both HRTs and plants significantly
Discussion
The removal percentages in the planted systems were 70.43–76.38% for DOC, 76.16–91.83% for ammonia-N, 47.89–63.37% for inorganic-N, 75.15–79.06% for total Kjeldahl nitrogen, >97% for ortho-phosphate and 86.65–91.83% for total phosphorus. These removal efficiencies were comparable to and even higher than that reported by previous studies of commonly used perennial wetland plants (Table 4). The quality of most treated effluent was able to meet the discharged standards set the Hong Kong Government
Conclusions
From the 6-month wastewater treatment study, it is clear that the mangrove microcosm with intermittent subsurface horizontal flow is effective in removing organic matter, nitrogen and phosphorus from primary settled municipal wastewater. The treated effluents, in terms of all forms of nitrogen and inorganic phosphate, were able to meet the effluent discharge standards for inland waters set by the Hong Kong Environmental Protection Department. The planted mangroves had higher treatment
Acknowledgement
The work described in this paper was supported by the Areas of Excellence Scheme established under the University Grants Committee of the Hong Kong SAR, China (Project No. AoE/P-04/2004).
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